Transport for London today announced details of the tender for a free afternoon and evening newspaper on the London underground.

Newspaper groups have until April 21 to express interest in establishing a free rival to Associated Newspaper's London Evening Standard, according to TfL's tender published in the Official Journal of the European Union.

The Office of Fair Trading last month cleared the way for the free newspaper after Associated, which also publishes the Daily Mail, agreed to give up its exclusive rights to the afternoon and evening free distribution slots at the 240 stations on the tube network.

Associated will continue to publish its free weekday morning title Metro, and could bid for the afternoon newspaper slot to protect the Standard.

Other publishers to express interest in the tender include Associated's rival, Richard Desmond's Northern & Shell, as well as Rupert Murdoch's News International and Guardian Media Group, the owner of MediaGuardian.co.uk.

It was Mr Desmond's complaints about Associated's monopoly of distribution points on the tube in 2003 that led to the OFT investigation.

But it is uncertain how many interested parties will be willing to commit resources to the project at a time when the advertising market is weak.

Richard Parry, the director of strategy and service development for London Underground, said: "This tender means not just more choice for passengers on the tube. Any additional money generated from a new contract will be invested into London's transport system driven by TfL's £10bn five-year investment programme."

Ken Livingstone, the mayor of London, hopes a new newspaper will rival the Standard, one of his fiercest critics, and has said a new contract could raise as much as £10m for the underground.

But many companies will balk at have to pay such a high price to gain entry to tube distribution points.